Caustic regeneration process



Feb. 27, 1962 A. M. THOMAS, JR

CAUSTIC REGENERATION PROCESS Filed Jan. 14, 1959 3 uzoN zQEexo w o. J; 6EEEEE 305:. mm T 2 1. :75 @2308 Em; 4 Q :2

Arthur M. Thomas, Jr. Inventor By 'R 6-+3.?1M M% Agenf United StatesPatent 3,023,084 CAUSTlC REGENERATEUN PROtIESS Arthur M. Thomas, .lr.,Railway, N.J., assignor to Esso Research and Engineering Company, acorporation of Delaware Filed Jan. 14, 1959, Ser. No. 7863M 7 (Ilaims.(Cl. 23-484) The present invention is concerned with an improved methodof regenerating alkaline solution which has been employed to wash ahydrocarbon oil containing mercaptans and phenolic constituents. Moreparticularly, it deals with a combination process whereby both extractedmercaptans and phenolates may be removed from the spent alkalinesolution, thus liberating alkali for further oil extraction.

The use of an alkaline wash solution to treat hydrocarbon oils is Wellknown in the art. Hydrocarbon fractions such as LPG, naphthas, andheating oils and light and heavy catalytic naphthas, generally boilingin the range of 0 to 600 F., contain mercaptans and othersulfur-containing compounds. Mercaptans are particularly undesirable dueto their noxious odor, and sulfur-containing materials are in generaldeleterious since they adversely afiect stability as well as combustionproperties. Sulfur removal is also necessary in cases where material isto be fed to some catalytic reforming or polymerization process wheresmall amounts of sulfur are catalyst poisons. Sulfur in naphtha lowerslead response; in heating oils'it burns to form corrosive gases. Bysubjecting such oils to extraction with an alkaline solution, e.g.sodium or potassium hydroxide, mercaptans and sulfur-containingcompounds are removed into the alkaline phase. Since the feedhydrocarbon oils invariably also contain phenolic constituentscharacterized by para tertiary amyl phenol, the alkaline extract willalso contain phenolates. Gradually, the concentration of extractedmaterials increases and it is periodically necessary to replace thealkaline solution with fresher material. Due to the large volumes ofalkali involved, it is economically necessary that the spent alkalinesolution containing mercaptides, phenolates, sulfides, etc. beregenerated.

Numerous means have been advanced in the art for regenerating suchalkaline solutions. Since the primary purpose of the alkaline Wash isthe removal of mercaptans and sulfides, the prior art processes areprincipally concerned with removing these materials.

In one such well known process, it has been suggested to subject thespent alkaline solution to steam stripping at pressures of 50 p.s.i.g.and less, and temperatures of 100 to 300 F. It has been shown that thesemildstripping conditions are effective in removing mercaptides from thespent solution.

Competitive with the above process is the oxidative regenerationprocess. This process accomplishes the same purpose as the mildstripping in that it removes mercaptides. The mercaptides are oxidizedto disulfides which are readily separated from the alkaline solution.

Both conventional steam stripping step and conventional oxidativeregeneration have little or no effect on phenolate removal.

Though the above processes have individually met with some success, theyhave encountered difiiculties. In spite of regeneration by thesemethods, ultimately concentrations of carboxylic acids and phenolates aswell as other materials prevent indefinite regeneration and use ofregenerated solution. These compounds tie up the free sodium ionsleaving none to react with and extract the phenolates, mercaptans,sulfides and acid oils from the hydrocarbon stream. The resultingalkaline solution disposal problem is'quite' severe since due to itshigh phenolic content it is detrimental to aquatic life and thusnormally can not be simply discharged into streams, lakes, or the like.The increasing numbers of municipal restrictions regarding wastedisposal have aggravated this matter. Relatively costly means ofreducing phenol content in spent solution, such as by a biologicaloxidation process, have been reverted to in attempts to reduce pollutionupon discharging spent alkaline Wash solutions. Another means suggestedfor reducing this problem is the use of carbon dioxide so as to springphenolates out of solution by converting them to phenols. However, thelatter process has severe limitations since the treatment of spentcaustic with carbon dioxide as proposed in the prior art also Willconvert free caustic to carbonates thus. depleting free alkali. Theresultant buildup of carbonates necessitates ultimate discard of thecaustic.

Thus there exists a need in the art for a process whereby the abovedifiiculties may be substantially eliminated. The present inventionserves to satisfy this need.

In accordance With the present invention, spent alkali solution(containing mercaptans, phenolates, sulfides, etc.) is first subjectedto a severe stripping treatment at pressures above about 500 p.s.i.g.and a temperature of at least 400 F. Normally, stripping pressures willbe in the range of 500 to 2500 p.s.i.g. and temperatures of 400 to 700F. are employed. It has been found that these severe strippingconditions result in removal of phenolates due to their hydrolysis tophenols, the phenols being removed from the solution by the strippingaction of the steam. Although this severe treat serves primarily toremove phenolates, some sulfides as Well as mercaptans will also beremoved.

The treated solution having a substantially decreased concentration ofphenolates, e.g. less than 50 wt. percent of the concentration in theinitial spent alkaline solution, is then subjected to a fairlyconventional oxidative regeneration step such as treatment with air atmoderate pressure and temperature.

The present invention offers numerous advantages. It has been found thatthe prior severe stripping treat not only serves to partially regeneratethe caustic, but by having removed phenolates and some sulfides prior tooxidation, the oxidation step itself is enhanced. The mercaptanoxidation rate is substantially greater at reduced concentration ofphenolates, as Well as being greater at reduced concentration ofsulfides. Further, the amount of alkali metal thiosulfate normallyformed from sulfides in the oxidation step is reduced thereby reducingthe loss of alkali metal held in this manner.

In addition to improving the oxidation step, the phenolate concentrationin the recycled alkaline wash is reduced as is the concentration ofphenolics in the solution ultimately to be sent to disposal. Further,the phenols removed from the severe stripping treatment may be recoveredand used for the manufacture of chemicals among them phenolic typeresins, plastics and plasticizers. The stripping step does not spendalready present tree alkali but treats only those alkali metal ionswhich are combined with the contaminating organic molecules.

It should be clearly understood that the present process isdistinguished from simply employing a severe heat soaking of spentalkaline solution. The present process requires the use of a strippinggas, such as steam. If no stripping gas is supplied, only a small amountof the phenolates, e.g. less than about 20%, are removed. In the absenceof stripping gas, phenolates are simply held in the Vapor space abovethe solution, thus building up to an equilibrium with the phenolates inthe liquid phase and only a small degree of phenolate removal isobtained. Secondly, the present invention contemplates a two stepregeneration in which mercaptideremoval is sacrificed in the first stepin favor of phenolate removal, the oxidative regeneration stepthereafter efiiciently serving to reduce mercaptide concentration todesired levels.

Similarly, conventional mild stripping conditions do not serve to removephenolates, but rather are an alternative toprior art oxidationprocedures.

By way of clarifying nomenclature, the term alkaline solution denotesthe basic solutions (as opposed to acidic) formed from the hydroxides orsalts of the alkali metals, particularly those of sodium and potassium.The term oxidative regeneration includes the various prior artprocedures wherein mercaptides are reacted with oxygen to form'disulfides. The oxygen may be pure oxygen, air, ozone, etc.

The various aspects and modifications of the present invention will bemade more clearly apparent by reference to the following description,examples and accompanying drawing. 1

The drawing is a simplified illustration of the present systemconsisting principally of stripper and oxidation zone 24. By way ofsupplying a specific embodiment, it is desired to regenerate the spentcaustic solution resulting from caustic washing of a heavy catalyticnaphtha fraction with an initially 50 B. sodium hydroxide.

The spent caustic fed to unit 10 through line 11 contains 4 wt. percentphenols and based on milligram per 100 cc. of caustic it has a mercaptannumber of 91 and a sulfide number of 16. The phenols are in the form ofsodium phenolates. Substantially all the contaminants were initiallyderived from the treated hydrocarbon oil.

Stripping zone 10 operates at a pressure of 600 p.s.i.g. and atemperature of 500 F. A stripping gas such as steam is introduced intothe lower portion of zone 10 via line 12, and serves to strip phenolsfrom the spent caustic solution, phenols being removed overhead throughoutlet 18 along with the stripping gas. Additionally, some sulfides andmercaptides will also be removed by the stripping gas. About pounds ofsteam are employed per pound of spent alkaline solution treated in unit10, stripping being etiectecl for a period of about 4 hours. The contacttime is capable of Wide variation, it normally being adjusted to stripout at least 50 wt. percent of the phenols which are initially in thespent solution in the form of phenolates.

The bulk of the caustic is withdrawn through line 13. its composition istabulated below:

The stripped alkaline solution is then circulated to oxidation zone 24by means of pump 14 and lines 15 and 17. If desired, a portion may berecycled to unit 19 through conduit 16.

Oxidation zone 24 may be any one of a number of conventional oxidativeregeneration processes such as air blowing at moderate or elevatedpressures at moderate or elevated temperatures. In the presentembodiment, zone 24 operates at about 120 F. and 40 p.s.i.g. Air orother oxygen-containing gas is supplied through line 25 and serves toconvert mercaptides to disulfides. In addition sulfides are converted.to thiosulfates. Gases are vented through line 22, preferably into thesame outlet system employed for venting the stripping zone gases, aswill be later described.

Oxidized caustic solution is passed to separation drum 27 by conduit 26.Disullides may normally be readily separated by gravity settling, thelighter disulfide layer being removed through outlet 28. Regeneratedcaustic is Withdrawn through line 29, and may be recycled for furtherhydrocarbon oil extraction.

The regenerated caustic in line 29 now has a mercaptan No. of less than5.0 and a sulfide No. of less than 5.0. The oxidation step hassubstantially no eitect on the concentration of phenols.

Returning to the stripping gas-phenols eifiuent of zone 10, phenols mayreadily be separated by cooling the stream in unit 19. Phenols andsulfides are withdrawn through outlet 20 for further processing or aresimply collected. The separated stripping gas may be vented throughlines 21 and 23, or if desired may be recycled to the stripping zone, bymeans not shown.

In many cases, it Will be desirable to heat exchange the stripping gassent to unit 11 with the hot etfiuent solution withdrawn through line 13in order to conserve heat.

Various modifications may be made to the system described. For example,only a part of the total spent alkaline solution might be subjected tothe present two-step rocess while the remaining solution is simplyregenerated by conventional means, the two fractions thereafter beingcombined for hydrocarbon extraction. As the spent alkaline solutioncomposition changes due to refinery operational changes or as pollutionregulations become more severe, the fraction of spent solution treatedin the present manner may be varied. Further, although steam ispreferred for use as the stripping gas, other gases such as nitrogen,flue gases, etc., may be utilized.

Various experiments will now be detailed in order to illustrate theoperation of the present invention and the advantageous results to berealized thereby.

Example 1 Tabulated below are the results of subjecting various spentcaustic solutions which had previously been used forhydrocarbonextractions to the present high pressure, high temperature strippingtreatment.

TABLE 2 HIGE PRESSURE STRIPPING OF SPENT ALKALINE SOLUTION Operatingconditions:

Temperature, F. 500 179 496 553 60" 610 608 Pressure, p.s.i.g.. 600 600600 1,000 1, 500 1, 500 1, 500 Steaming rate, Min... 0. 99 0.98 0.991.02 1.0 1.00 0. 51 Water inlet temperature, F 200 201 200 201 200 200201 Phenoiate concentration,

weight percent' Initial".-. 20. 4 9. 0 5. 3 9. 0 9. 0 20. 4 20. 4 After1 non 18. 9 7. 7 4. 7 7. 7 7. 5 17.7 After 3 hours. 14.0 6.6 3. 4 5. 85. 4 11.0 11. 0 After 5 hours 10.0 4. 6 1. 5 3. 6 3. 4 7. 7 7. 0Attertlhours 8.5 4.0 1.6 3.3 2.9 3.7 6.1 Percent removed.. 58.4 63.369.8 67.0 71.0 81.8 70.0

Mercaptan number,

rugs/100 cc;

n' 1 560 388 5 388 388 560 560 After 6 hours... 264 127 61 144 150 280280 Percent remove 52. 9 07.3 41. 9 62.9 61. 3 50.0 50.0

Sulfide number, rugs/100 Initial 140 70 35 70 70 140 M0 After 6 hours 586 54 60 106 Percent removed...- 39. 3 17. 2 82.8 22.9 14. 3' 24. 3 28.1Caustic concentration,

weight percent:

Initial 3. 8 7. 6 7. 6 14. 4 14. 1 After 6 hours. 9. 2 15. 2 14.1 23.818.2 Percent incre 142.1 100.0 85. 5 65.3 26. 4 Steam consumption,1b./1b. of spent NaOH solution 41. 3 33. 5 73. 3 53. 6 61.1 42. 5 53. 7

The various experimental runs depicted in Table 2 illustrate that thepresent high pressure-high temperature stripping treatment substantiallyreduces the concentration of phenolates. Further, it reduces theconcentration of sulfides in the alkaline solution and at leastpartially regenerates the solution.

It is also to be noted that under the conditions of the stripping'step asubstantial portion of the mercaptides will remain in the alkalinesolution.

Example 2 To illustrate the difierence between the present strippingstep and steam stripping under conventional mild conditions, spentcaustic was subjected to steam stripping under the conditions of theprior art.

Table 3, particularly when compared to Table 2, shows that conventionalstripping does not remove appreciable quantities of phenols, but ratherconditions are such as to favor sulfide removal. Thus, the present hightemperature stripping is readily distinguished from the practices of theprior art.

Example 3 The data presented below show the benefits of employing thepresent stripping operation prior to a conventional oxidativeregeneration treatment.

The results in the following table are the mercaptide oxidation rates ata mercaptide level of 1000'mgs./ 100 cc. of solution for two levels ofphenolate concentration. The data are from the air oxidation of aninitially 25 B. caustic.

TABLE 4 Temperature, Oxidation Phenolates, vol. percent F. rate,rugs/100 ccJhr.

It is thus seen that the oxidation rate in the absence of phenolates isnearly six times greater than when the spent caustic contains 36 vol.percent phenol (present in the form of phenolates). This is in spite ofthe fact that the temperature is 23 F. lower in the case of lowphenolate concentration.

The above results show the advantage of reducing the phenolate contentof the spent alkaline solution prior to subjecting it to oxidativeregeneration, as is done by the present combination process.

Similarly, Table 5 illustrates that for approximately the same mercaptanlevel, reducing the amount of sulfide compoundspresent in the solutionto be subjected to air oxidation will enhance the rate of oxidation. Thedata was obtained from the air oxidation of an originally 18 B. spentcaustic at 100 F. and 50 p.s.i.g. in a stirred vessel. No phenolateswere present.

TABLE 5 Sulfide Oxidation Mercaptlde number, mgs./l00 cc. number rate,mgs./100

mgs./100 cc. cc. r.

The above data show that the mercaptide oxidation rate is appreciablygreater at low concentration of sulfides. Thus, there is a furtherimprovement in the oxidation step because of the previous strippingwhich had reduced sulfide concentration. (See Table 2.)

summarily, it is seen that the present combination process offerssubstantial advantages over both conventional steam stripping and directoxidation of spent alkaline solutions. That which is sought to beprotected is set forth in the following appended claims.

What is claimed is:

1. An improved process for regenerating a spent alkaline solution whichhas previously been employed to wash a hydrocarbon oil containingmercaptans and phenolic materials which comprises, in combination;subjecting said spent alkaline solution to stripping with a gas at apressure of at least 500 p.s.i.g., and a temperature of at least 400 F.so as to remove a substantial portion of the phenolates from said spentalkaline solution; thereafter removing mercaptans from said thus treatedalkaline solution by means of an oxidative regeneration step whereinsaid mercaptans are converted to disulfides by reaction with oxygen.

2. The improved process of claim 1 wherein said spent alkaline solutionis subjected to stripping at a temperature in the range of 400 to 700 F.anda pressure in the range of 500 to 2500 p.s.i.g., and steam isemployed as the stripping gas.

3. The improved process of claim 1 wherein said spent alkaline solutionis a spent solution of sodium hydroxide, and wherein at least 50 weightpercent of the phenolates in said spent sodium hydroxide solution areremoved by said high pressure stripping prior to passing said solutionto said oxidative regeneration step.

4. The improved process of claim 1 wherein an oxygencontaining gas isemployed in said oxidative regeneration step to convert mercaptans todisulfides.

5. The improved process of claim 1 wherein said spent alkaline solutionis a spent caustic solution.

6. In the oxidative regeneration process for regenerating spent alkalinesolution which has been employed to treat a hydrocarbon oil containingmercaptans and thiophenols, the improvement which comprises, gasstripping said alkaline solution at a pressure in the range of 500 to2500 p.s.i.g., and a temperature in the range of 400 to 700 P. so as toremove at least 50 wt. percent of the phenolates present in said spentalkaline solution prior to subjecting said alkaline solution tooxidative regeneration wherein mercaptans are converted to disulfides byreaction with oxygen.

7. The improvement of claim 5 wherein steam is employed for gasstripping said alkaline solution and said alkaline solution is spentcaustic.

References Cited in the file of this patent UNITED STATES PATENTS Happelet al Aug. 5, 1952 Petty Dec. 2, 1958 OTHER REFERENCES

1. AN IMPROVED PROCESS FOR REGENERATING A SPENT ALKALINE SOLUTION WHICHHAS PREVIOUSLY BEEN EMPLOYED TO WASH A HYDROCARBON OIL CONTAININGMERCAPTANS AND PHENOLIC MATERIALS WHICH COMPRISES, IN COMBINATION;SUBJECTING SAID SPENT ALKALINE SOLUTION TO STRIPPING WITH A GAS AT APRESSURE OF AT LEAST 500 P.S.I.G., AND A TEMPERATURE OF AT LEAST 400*F.SO AS TO REMOVE A SUBSTANTIAL PORTION OF THE PHENOLATES FROM SAID SPENTALKALINE SOLUTION; THEREAFTER REMOVING MERCAPTANS FROM SAID THUS TREATEDALKALINE SOLUTION BY MEANS OF AN OXIDATIVE REGENERATION STEP WHEREINSAID MERCAPTANS ARE CONVERTED TO DISULFIDES BY REACTION WITH OXYGEN.